1. Field of the Invention
The present invention generally relates to motor control and, more particularly, to a motor control apparatus and an image forming apparatus that drive a stepping motor in a plurality of excitation modes.
2. Description of the Related Art
An image forming apparatus such as a copying machine conveys recording paper in a speed suitable for each of various types of the recording paper or printing modes. For example, when thick paper is conveyed, the thick paper is conveyed at a speed half of a conveyance speed of plain paper. To drive rollers for conveying the recording paper, a stepping motor is often used. When the stepping motor is driven in two-phase excitation, high torque can be obtained. However, in the two-phase excitation, when the stepping motor is driven at a low speed, significant vibration occurs. To solve the problem, Japanese Patent Application Laid-Open No. 62-002895 discusses to drive a stepping motor in two-phase excitation when the motor is driven at a high speed and to drive the motor in one-two phase excitation when the motor is driven at a low speed.
However, when the stepping motor is driven in the two-phase excitation at the high speed and the motor is driven in the one-two phase excitation at the low speed, the following problems occur.
When a motor driver drives the stepping motor, the motor driver starts to excite a stator in a predetermined excitation pattern. When a rotor of the stepping motor is not positioned at an angle corresponding to an initial position in the excitation pattern, if the excitation is started in the predetermined excitation pattern, the rotor cannot follow the excitation of the stator and causes a loss of synchronization and vibration.
Such a phenomenon may occur when the excitation pattern is switched from the two-phase excitation to the one-two phase excitation or from the one-two phase excitation to the two-phase excitation at arbitrary timing.
When the stepping motor is driven, stopped, and restarted without switching an excitation method, the loss of synchronization and vibration can be prevented if the excitation phase at the time of the driving stop is stored and the excitation is restarted from the stored excitation phase. However, when the excitation method is switched, the excitation phases of before and after the switching do not correspond to one to one. Accordingly, the excitation cannot be restarted in the excitation method different from the previous excitation method by only storing the excitation phase at the time of the driving stop.
Especially, when an excitation phase at the time of the driving stop in the one-two phase excitation is a phase 1, and the excitation phase is switched to the two-phase excitation, the excitation phase (phase 1) at the driving stop does not correspond to the excitation phase (phase 2) at the driving restart. Accordingly, it is not possible to determine which phase is to be used as the phase 2 only from information about the excitation phase at the driving stop.